Bump-on-trace (BOT) structures

ABSTRACT

A bump-on-trace (BOT) structure is described. The BOT structure includes a first work piece with a metal trace on a surface of the first work piece, wherein the metal trace has a first axis. The BOT structure further includes a second work piece with an elongated metal bump, wherein the elongated metal bump has a second axis, wherein the second axis is at a non-zero angle from the first axis. The BOT structure further includes a metal bump, wherein the metal bump electrically connects the metal trace and the elongated metal bump. A package having a BOT structure and a method of forming the BOT structure are also described.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 13/095,185, entitled “REDUCED-STRESS BUMP-ON-TRACE(BOT) STRUCTURES,” filed on Apr. 27, 2011, which is incorporated hereinby reference in its entirety.

The present application is related to U.S. application Ser. No.13/035,586, entitled “EXTENDING METAL TRACES IN BUMP-ON-TRACESTRUCTURES,” filed on Feb. 25, 2011, which is incorporated herein byreference in its entirety.

BACKGROUND

Bump-on-Trace (BOT) structures have been used in flip chip packages,wherein metal bumps are bonded onto narrow metal traces in packagesubstrates directly, rather than bonded onto metal pads that havegreater widths than the respective connecting metal traces. The BOTstructures require smaller chip areas, and the manufacturing cost of theBOT structures is relatively low. However, there are technicalchallenges related to BOT structures.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1A and 1B illustrate cross-sectional views of a package structurein accordance with an embodiment.

FIG. 1C shows a top view of a copper post, in accordance with someembodiments.

FIG. 1D illustrates an exemplary perspective view of the metal packagestructure of FIGS. 1A and 1B, in accordance with some embodiments.

FIG. 1E shows the package structure of FIGS. 1A and 1B with an axispointing to the center O of a die, in accordance with some embodiments.

FIG. 1F shows a cross-sectional view of the package structure of FIG.1E, in accordance with some embodiments.

FIG. 2A shows a number of bump-on-trace (BOT) structures on a die, inaccordance with some embodiments.

FIG. 2B shows a table of data comparing the highest stresses in adielectric layer and the metal trace of each BOT structure of FIG. 2A,in accordance with some embodiments.

FIGS. 3A and 3B illustrate cross-sectional views of another packagestructure in accordance with an embodiment.

FIG. 3C shows a top view of the BOT structure of FIGS. 3A and 3B, inaccordance with some embodiments.

FIG. 3D shows the BOT structure of FIGS. 3A and 3B with an axis of themetal trace pointing to the center of a die, in accordance with someembodiments.

FIG. 3E shows a table of normalized stress simulation results comparingtwo BOT structures, in accordance with some embodiments.

FIG. 4A shows 2 BOT structures on two locations on a die, in accordancewith some embodiments.

FIG. 4B shows a table of stress simulation results comparing stresses ofBOT structures at different locations, in accordance with someembodiments.

FIG. 5A shows a metal bump with an axis at an angle with the axis of ametal trace, in accordance with some embodiments.

FIG. 5B shows examples of different shapes of elongated metal bumps overa metal trace, in accordance with some embodiments.

FIG. 6 shows a process flow of reducing stresses of BOT structures on apackaged substrate, in accordance with some embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative, and do not limit the scope of the disclosure.

A package structure comprising a Bump-on-Trace (BOT) structure isprovided in accordance with an embodiment. The variations of theembodiment are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

FIG. 1A illustrates a cross-sectional view of a package structure (orbump-on-trace structure) 150 in accordance with an embodiment. Thepackage structure 150 includes work piece 100 bonded to work piece 200.Work piece 100 may be a device die that includes active devices such astransistors (not shown) therein, although work piece 100 may also be aninterposer that does not have active devices therein. In an embodimentwherein work piece 100 is a device die, substrate 102 may be asemiconductor substrate such as a silicon substrate, although it mayinclude other semiconductor materials. Interconnect structure 104, whichincludes metal lines and vias 106 formed therein and connected to thesemiconductor devices, is formed on substrate 102. Metal lines and vias106 may be formed of copper or copper alloys, and may be formed usingdamascene processes. Interconnect structure 104 may include a commonlyknown inter-layer dielectric (ILD, not shown) and inter-metaldielectrics (IMDs) 108. IMDs 108 may comprise low-k dielectricmaterials, and may have dielectric constants (k values) lower than about3.0. The low-k dielectric materials may also be extreme low-k dielectricmaterials having k values lower than about 2.5.

Work piece 100 may further include under-bump metallurgy (UBM) layer 110and a copper post 112 on UBM layer 110. Throughout the description, thecopper post 112 is also referred to as a copper-containing bump or metalbump. Although copper post 112 is used as an example in the descriptionhere and below, other types of metal bumps, such as solder bumps, mayalso be used in place of copper post 112. The UBM layer 110 is disposedon a metal pad 105, which is part of interconnect structure 104. Betweenthe interconnect structure 104 and the UBM layer 110 not contacting themetal pad 105, there is a passivation layer 107. In some embodiments,the passivation layer 107 is made of polyimide.

Work piece 200 may be a package substrate, although it may be otherpackage components such as interposers, for example. Work piece 200 mayinclude metal lines and vias 202 connecting metal features on oppositesides of work piece 200. In an embodiment, metal trace(s) 210 on thetopside of work piece 200 are electrically connected to ball grid array(BGA) balls 212 on the bottom side of work pieces 200 through metallines and vias 202. Metal lines and vias 202 may be formed in dielectriclayers 214, although they may also be formed in a semiconductor layer(such as a silicon layer, not shown) and in the dielectric layers thatare formed on the semiconductor layer.

Metal trace 210 is formed over a top dielectric layer in dielectriclayers 214. Metal trace 210 may be formed of substantially pure copper,aluminum copper, or other metallic materials such as tungsten, nickel,palladium, gold, and/or alloys thereof. FIG. 1A shows that the copperpost (or metal bump) 112 has a length of L₁, in accordance with someembodiments. FIG. 1A also shows that the metal trace 210 has a lengthL₂, in accordance with some embodiments.

Work pieces 100 and 200 are bonded to each other through solder bump220, which may be formed of a lead-free solder, a eutectic solder, orthe like. Solder bump 220 is bonded to, and contacts, the top surfacesof metal trace 210 and copper post 112,

FIG. 1B illustrates a cross-sectional view of the package structure 150shown in FIG. 1, wherein the cross-sectional view is obtained from theplane crossing line 2-2 in FIG. 1A. As shown in FIG. 1B, solder bumps220 may also contact the sidewalls of metal trace 210. After the bondingof work pieces 100 and 200, a mold underfill (MUF) (not shown) may befilled into the space between work pieces 100 and 200, in accordancewith some embodiments. Accordingly, a MUF may also be filled into thespace between neighboring metal traces 210. Alternatively, no MUF isfilled, while air fills the space between work pieces 100 and 200, andfills the space between neighboring metal traces 210. FIG. 1B shows thatthe copper post (or metal bump) 112 has a width of W₁, in accordancewith some embodiments. FIG. 1B also shows that the metal trace 210 has awidth W₂, in accordance with some embodiments.

FIG. 1C shows a top view of the copper post 112, in accordance with someembodiments. The copper post 112 has a shape of an oval with a width W₁and a length L₁. In some other embodiments, the ratio of L₁/W₁ isgreater than 1. In some embodiments, the ratio of L₁/W₁ is equal to orgreater than about 1.2. In some embodiments, the L₁ is in a range fromabout 10 μm to about 1000 μm. In some embodiments, W1 is in a range fromabout 10 μm to about 700 μm.

FIG. 1D illustrates an exemplary perspective view of metal trace 210,the overlying copper post 112, and solder bump 220, in accordance withsome embodiments. The metal trace 210 has a width W₂, and a length L₂.In some other embodiments, the ratio of L₂/W₂ is greater than 1. In someembodiments, the ratio of L₂/W₂ is greater than about 1.2. In someembodiments, the L₂ is in a range from about 10 μm to about 10,000 μm.In some embodiments, W₂ is in a range from about 10 μm to about 500 μm.The structure as shown in FIG. 1D is referred to as being a BOTstructure, because solder bump 220 is formed directly on the top surfaceand sidewalls of metal trace 210, and not on a metal pad that has awidth significantly greater than width W₂ of metal trace 210. In someembodiments, the ratio of W₁/W₂ is in a range from about 0.5 to about 5.

FIG. 1D also shows that the metal trace 210 has an axis line X-X, inaccordance with some embodiments. Axis X-X is defined along the lengthof the metal trace. The copper post 112 has an axis line X′-X′, inaccordance with some embodiments. Axis X′-X′ is defined along the lengthof the copper post 112. As shown in FIG. 1D, axis X′-X′ is substantiallyparallel to axis X-X. Therefore, the BOT structure 150 of FIG. 1D has anaxis X-X. The direction the BOT structures, such as BOT structure 150,point to the center of the die and the relative locations of the BOTstructures to the center of the die can affect the stress exerted on thestructures.

FIG. 1E shows the BOT structure 150 with the axis X-X pointing to thecenter O of a die, in accordance with some embodiments. FIG. 1F shows across-sectional view of BOT structure 150, in accordance with someembodiments. Stress simulation (or mechanical analysis) results showhigh stress at location M of the dielectric sub-layer (noted as layer108′ in FIG. 1F), which is a sub-layer of the IMDs 108 and contacts themetal pad 105, as shown in FIGS. 1E and 1F. The stress simulation (ormechanical analysis) is performed by using ANSYS 12.1 simulator, whichis made by ANAYS, Inc. of Canonsburg, Pa. As mentioned above, IMDs 108may comprise low-k dielectric materials, and may have dielectricconstants (k values) equal to or lower than about 3.0 or may also beextreme low-k (ELK) dielectric materials having k values equal to orlower than about 2.5. As a result, the dielectric sub-layer 108′contacting the metal pad 105 may be made of a material having adielectric constant (k value) equal to or lower than about 3.0 or mayalso be an extreme low-k (ELK) dielectric material having a k valueequal to or lower than about 2.5. A porous extreme low-k (ELK) materialwith a k value of about 2.5 is used in the simulation. The stress of thedielectric sub-layer 108′ at location M is highest due to location Mbeing farthest away from the die center “O.” Stress simulation resultsalso show high stress at location N of metal trace 210, as shown inFIGS. 1E and 1F. Location N of metal trace 210 is closest to the centerof the die.

The axis direction and relative position of a BOT structure affects thestress on the BOT structure. For example, if the axis of a BOT structureis pointed perpendicularly to the center of a die, the stresses on thedielectric layer 108′ and on metal trace 210 are higher than thestresses of the BOT structure shown in FIG. 1E. FIG. 2A shows BOTstructures, 251-256, on a die 250, in accordance with some embodiments.These BOT structures 251-256 all have structures similar to them thatare nearby and they are not isolated structures. Further, there areother structures in the remaining areas of die 250 that are not shown inFIG. 2A. BOT structures 251-256 are placed on locations A-F of die 250respectively. Both locations A and B are placed near two of the farcorners of die 250. Locations C-F are placed on 4 corners of a centerregion 258. The axes of BOT structure 251 at location A, BOT structure253 at location C, BOT structure 256 at location F all point toward thecenter P of die 250. In contrast, the axes of BOT structure 252 atlocation B, BOT structure 254 at location D, and BOT structure 255 atlocation E are all pointing perpendicularly to the center P of die 250.The BOT structures 251-256 of FIG. 2A are all similar to BOT structure150 of FIG. 1D with the axes of the metal bumps substantially parallelto the axes of the metal traces.

FIG. 2B shows a table of data comparing the highest stresses in thedielectric layer 108′ and the metal trace 201 of each BOT structure ofFIG. 2A, in accordance with some embodiments. FIG. 2B shows that thedielectric stress for BOT structure 251 at location A being the highest(110.0 MPa), which is followed by the BOT structure 252 at location B.The highest dielectric stresses of BOT structures 253-256 at locationsC-F, which are closer to the center P of die 250 are all smaller than(or about 60% of) the stresses of edge BOT structures 251 and 252. FIG.2B also shows that stress at the metal trace is highest for BOTstructure 252 at corner B (185.0 MPa), which is followed by the stressfor BOT structure 251 at location A. Stress of the metal trace of BOTstructure 252 at location B is about 40% higher than the stress of themetal trace of BOT structure 251 at location A. Because both structuresare placed near the corner edges, the much higher stress is due to theorientation of the BOT structures. The axis of BOT structure 252 (atlocation B) is pointed about perpendicularly to the center P of die 250.In contrast, the axis of BOT structure 251 is pointed toward (or aboutparallel) to the center P of die 250. The different orientations of theaxes of these two structures contribute to the significant difference instresses of metal traces. The data collected in FIG. 2B assume thesolder bumps 220 have been reflowed at 250° C. and then cooled to roomtemperature (25° C.).

The higher stresses on the metal traces of BOT structures with axespointing in directions substantially perpendicular to the center P ofdie 250 compared to BOT structures with axes pointing toward the centerP of die 250 are also supported by data of structures 253-256 atlocations C-F. The axes of BOT structures 254 and 255 at locations D andE respectively point perpendicularly to the center P of die 250. Incontrast, the axes of BOT structures 253 and 256 at locations C and Frespectively point toward the center P of die 250. The stress resultsshow that the highest stresses on the metal traces of BOT structures 254and 255 are higher (about 12% higher) than the highest stresses on themetal traces of BOT structures 253 and 256. Therefore, the metal tracestress results of BOT structures 253-256 also support the effect oforientation of axes of BOT structures.

Further, the results in FIG. 2B show that the stresses (both thedielectric stress and stress on metal traces) are higher when the BOTstructures are farther away from the die center. The stress results ofBOT structures 251 and 252, which are placed at corners farthest fromthe center P of die 250, are higher than the stress results of BOTstructures 253-256, which are placed closer to the center P of die 250compared to BOT structures 251 and 252. The extreme high stress on metaltrace of BOT structure 252 at location B could cause the metal trace,which is similar to metal trace 210 of FIGS. 1A, 1B, 1D and 1F, to belifted off from the substrate surface (delamination) and to disruptelectrical connection. Similarly, the stress on the metal trace of BOTstructure 251 at location A is also quite high and may also causedelamination of the metal trace. Further, the stress at the dielectriclayer of BOT structures 251 and 252 are also high relative to the stressat the other BOT structures. High stress at the dielectric layer, whichis similar to dielectric layer 108′ of FIG. 1F, of BOT structures couldalso cause interface delamination, which could be a reliability and/oryield issue. Therefore, it's important to seek solutions to reducestresses at dielectric layer over the metal pad and at metal traces forBOT structures.

FIGS. 3A and 3B illustrate cross-sectional views of the packagestructure 150′ in accordance with an embodiment. The substrates 100′ and200′ in FIGS. 3A and 3B are similar to substrate 100 and substrate 200of FIGS. 1A-1D, with the exception that the orientation of the copperpost 112′, which is similar to copper post 112, and its correspondingUBM layer 110′ and solder bump 220′. All are turned 90 degrees (i.e.,90°). Copper post 112′ is one type of metal bump. Other types of metalbumps, such as a solder bump, may also be used in place of copper post112′. FIG. 3A shows that the width W₁ of the copper post 112′ is in thesame direction as the length L₂ of the metal trace 210. FIG. 3B showsthat the length L₁ of the copper post 112′ is in the same direction asthe width W₂ of the metal trace 210.

FIG. 3C shows a top view of BOT structure 150′, in accordance with someembodiments. The metal trace 210 has a width W₂, and a length L₂. FIG.3C shows that the copper post 112′, which has a width W₁ and a length L₁and an axis Y-Y, is disposed perpendicularly above metal trace 210. AxisY-Y is substantially perpendicular to axis X-X of metal trace 210. FIG.3D shows the BOT structure 150′ with the axis X-X of the metal tracepointing to the center O′ of a die, in accordance with some embodiments.FIG. 3E shows a table of normalized stress simulation results comparingBOT structure 150 of FIGS. 1D-1F and BOT structure 150′ of FIGS. 3C-3D,in accordance with some embodiments. The stress results of BOT structure150 are normalized to be 1 and the stress results of BOT structure 150′are compared against the corresponding stresses of BOT structure 150.The results in FIG. 3E show that the peeling stress and total stress ofBOT structure 150′ at the dielectric layer 108′ are about 32% lower thanBOT structure 150. The results also show that the peeling stress of themetal trace 210 for BOT structure 150′ is lower than the BOT structure150 by about 56%. The simulation results show a drastic reduction instresses at the dielectric layer and at the metal trace. As mentionedabove, high stress at the metal trace can cause poor or no electricalcontact and reduce yield. In addition, high stress at the dielectriclayer 108′ can result in reliability issue, which can also degradeyield. By orienting the axis of the copper post 112 to be perpendicularto the axis of the metal trace 210, the maximum stresses at thedielectric layer 108′ and metal trace 120 can be significantly reduced.

FIG. 4A shows 2 BOT structures, 251′ on location A′ and 252′ on locationB′, on a die 250′, in accordance with some embodiments. These 2 BOTstructures 251′, 252′, and locations A′, B′ are similar to structures251, 252, and locations A, B of FIG. 2A respectively, with the exceptionthat the metal bumps of BOT structures 251′ and 252′ are oriented to beperpendicular to the metal traces of these two BOT structures and alsothe metal trace of BOT structure 252′ is reoriented toward the center P′of die 250′. FIG. 4B shows a table of stress simulation resultscomparing stresses of structures 251′, 252′, 251, and 252, in accordancewith some embodiments. The results show an about 30% reduction indielectric stress and an about 40% reduction stress in metal stress forBOT structure 251′ at location A′ compared to BOT structure 251 atlocation A. The reduction is attributed to the change in the orientationof the metal bump, such as copper post 112, from being parallel to beingperpendicular to the metal trace. The results also show an about 14%reduction in dielectric stress and an about 35% reduction stress inmetal stress for BOT structure 252′ at location B′ compared to BOTstructure 252 at location B. The reduction is attributed to the changein the orientation metal trace pointing toward the center P′ of die 250′and the metal bump, such as copper post 112, being changed from beingparallel to being perpendicular to the metal trace.

The results in FIGS. 3E and 4B indicate significant benefits in stressreduction by placing the metal bumps perpendicularly to the underlyingmetal trace and also by pointing the axes of metal traces toward thecenter of die. Such arrangement is especially needed for BOT structuresnear the edges or corners of the die, such as structures 251 and 252,which have higher stress and are more likely to delaminate or havereliability issues. The BOT structures 251-256 and 251′-252′ are allstructures surrounded by other similar structures. Isolated bumpstructures and BOT structures are known to have higher stresses,compared to structures surrounded by other structures. Therefore,isolated structures can also benefit from the BOT structure designdescribed above.

One potential downside of such BOT design is the larger surface area (orreal-estate) required. With the lengthy side of the copper post (ormetal bump), such as copper post 112, instead of the narrower width ofthe copper post, being perpendicular to the metal trace (210),additional space (or surface area) is needed. Therefore, such designrequires additional surface area. If higher density of BOT structures isneeded, the design can be applied on BOT structures that are most atrisk of delamination or reliability issues. For examples, BOT structuresnear the edge of the die or isolated BOT structures have higher stressesthan other BOT structures.

The new BOT structures described FIGS. 3A-3D show that the axis of thecopper post 112′ is substantially perpendicular to the axis the metaltrace 210′. In some embodiments, the length L₁ of copper post 112′ isevenly divided on metal trace 210′ (or the center of copper post 112′ isaligned with the axis of metal trace 210′. However, the axis of thecopper post 112′ does not need to be substantially perpendicular to theaxis of the metal trace 210′ to reduce the stress. FIG. 5A shows a topview of a copper post 112″ over a metal trace 210″. As described above,the copper post 112″ is at an angle “α” of about 90°, as shown in FIG.5A in accordance with some embodiments. In some embodiments, the angle“α” is in a range from about 30° to about 150°. In some embodiments, theangle “α” is in a range from about 45° to about 135°. In some otherembodiments, the angle “α” is in a range from about 60° to about 120°.

The BOT structures described above show that the top view of the metalbumps and their associated UBM layers are in the shape of an oval.Alternatively, the top view of the metal bumps can be in other elongatedshapes, such as a track-field-shaped oval (an oval with two parallelsides), a rectangle, a parallelogram, a trapezoid, or a triangle, etc,as shown in FIG. 5B in accordance with some embodiments. In someembodiments, the corners of the metal bumps are rounded to reducestress. Any elongated metal bumps could have the benefit of loweringstresses by placing the long axis not-parallel to the axis of metaltrace.

FIG. 6 shows a process flow 600 of reducing stresses of BOT structureson a packaged substrate, in accordance with some embodiments. Thepackage substrate is a packaged die. At operation 601, isolated BOTstructures and BOT structures at and/or near the edges or corners of thepackaged substrate are identified. Such BOT structures have highdelamination risks due to high stress at the metal trace interface andthe dielectric interface with the metal pad. At operation 603, the axesof metal traces of the BOT structures identified in operation 601 arealigned to point toward the center of the package structure (or packageddie) and/or the metal bumps of these BOT structures are designed to havetheir axes non-parallel to the axes of the metal traces on which theyare placed. In some embodiments, the axes of the metal bumps aresubstantially perpendicular to the axes of the metal traces to reducethe stresses of the BOT structures.

The embodiments of bump-on-trace (BOT) structures and their layout on adie described above reduce stresses on the dielectric layer on the metalpad and on the metal traces of the BOT structures. By orienting the axesof the metal bumps in non-parallel relation to the metal traces, thestresses can be reduced, which can reduce the risk of delamination ofthe metal traces from the substrate and the dielectric layer from themetal pad. Further, the stresses of the dielectric layer on the metalpad and on the metal traces may also be reduced by orienting the axes ofthe metal traces toward the center of the die. As a result, the yieldcan be increased.

In accordance with some embodiments, a bump-on-trace (BOT) structure isprovided. The BOT structure includes a first work piece with a metaltrace on a surface of the first work piece, wherein the metal trace hasa first axis. The BOT structure further includes a second work piecewith an elongated metal bump, wherein the elongated metal bump has asecond axis, wherein the second axis is at a non-zero angle from thefirst axis. The BOT structure further includes a metal bump, wherein themetal bump electrically connects the metal trace and the elongated metalbump.

In accordance with some other embodiments, a packaged substrate with abump-on-trace (BOT) structure is provided. The package includes a firstwork piece with a metal trace on a surface of the first work piece,wherein the metal trace has a first axis. The package further includes asecond work piece with a plurality of elongated bumps, wherein at leastone of the plurality of elongated metal bumps has a second axis and atleast another of the plurality of elongated metal bumps has a thirdaxis, wherein the second and the third axes are not the same and thesecond axis is at a non-zero angle from the first axis. The packagefurther includes a plurality of metal bumps electrically connecting theplurality of elongated bumps and the metal trace.

In accordance with yet other embodiments, a method of forming abump-on-trace (BOT) structure is provided. The method includes forming ametal trace on a first work piece, wherein the metal trace has a firstaxis. The method further includes forming an elongated metal bump on asecond work piece, wherein the elongated metal bump has a second axis,wherein the second axis is at a non-zero angle from the first axis. Themethod further includes electrically connecting the metal trace with theelongated metal bump using a metal bump.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, that performsubstantially the same function or achieve substantially the same resultas the corresponding embodiments described herein may be utilizedaccording to the disclosure. Accordingly, the appended claims areintended to include within their scope such processes, machines,manufacture, compositions of matter, means, methods, or steps. Inaddition, each claim constitutes a separate embodiment, and thecombination of various claims and embodiments are within the scope ofthe disclosure.

What is claimed is:
 1. A bump-on-trace (BOT) structure comprising: afirst work piece with a metal trace on a surface of the first workpiece, wherein the metal trace has a first axis, wherein the first workpiece is rigid, and an entirety of the metal trace is on the first workpiece; a second work piece with an elongated metal bump, wherein theelongated metal bump has a second axis, wherein the second axis is at anon-zero angle from the first axis; and a metal bump, wherein the metalbump electrically connects the metal trace and the elongated metal bump.2. The BOT structure of claim 1, wherein the elongated metal bump has afirst length extending in a first direction, the first length rangingfrom 10 microns (μm) to 1,000 μm.
 3. The BOT structure of claim 2,wherein the metal trace has a second length extending in a seconddirection, different from the first direction, the second length rangingfrom 10 μm to 10,000 μm.
 4. The BOT structure of claim 2, wherein theelongated metal bump has a first width extending in a second direction,different from the first direction, the first width ranging from 10 μmto 700 μm.
 5. The BOT structure of claim 2, wherein the metal trace hasa second width extending in the first direction, the width ranging from10 μm to 500 μm.
 6. The BOT structure of claim 1, wherein an anglebetween the first axis and the second axis ranges from 60-degrees to120-degrees.
 7. The BOT structure of claim 1, wherein the elongate metalbump has a first half length extending beyond a first edge of the metaltrace and a second half length extending beyond a second opposite edgeof the metal trace, wherein the first half length is equal to the secondhalf length.
 8. The BOT structure of claim 1, wherein the second workpiece is an interposer.
 9. The BOT structure of claim 1, wherein thefirst work piece and the second work piece comprise active devices. 10.A method of making a bump-on-trace (BOT) structure, the methodcomprising: forming a metal trace on a first work piece, wherein themetal trace has a first axis, the metal trace is formed completely onthe first work piece, and the first work piece is rigid; forming anelongated metal bump on a second work piece, wherein the elongated metalbump has a second axis, wherein the second axis is at a non-zero anglefrom the first axis; and electrically connecting the metal trace withthe elongated metal bump using a metal bump.
 11. The method of claim 10,wherein forming the elongated metal bump comprises forming the elongatemetal bump to have the second axis at an angle with the first axis, theangle ranging from 60-degrees to 120-degrees.
 12. The method of claim10, further comprising filling a space between the first work piece andthe second work piece with a mold underfill.
 13. The method of claim 10,wherein electrically connecting metal trace with the elongated metalbump comprises reflowing a solder bump.